Laser-radiation detectors (sensors) are used in laser applications wherein laser-radiation power needs to be measured or monitored. The power measurement may be required from simple record-keeping or as part of some closed loop control arrangement. Commonly used radiation detectors are based on either photodiodes or thermopiles.
The photodiode-based sensors detect laser-radiation by converting photon energy of radiation to be measured into an electron-hole pairs in the photo-diode thereby generating a corresponding current, which is used a measure of laser-radiation power. Photodiodes sensors have a relatively fast temporal response, with rise times typically less than 1 microsecond (μs). A disadvantage of photodiode detectors is a limited spectral response. This spectral response is determined by the particular semiconductor materials used for forming the photodiode. By way of example, photodiode sensors based on silicon have a spectral acceptance bandwidth between about 0.2 micrometers (μm) and about 2.0 μm. A second limitation of a photodiode is relatively low optical power saturation. Photodiodes are typically limited to direct measurement of laser powers of less than 100 milliwatts (mW).
Thermopile sensors include a solid element which absorbs the radiation, thereby heating the element. One or more thermocouples in contact with the element create a current or voltage representative of the laser-radiation power incident on the element. Thermopile sensors have a slow response time relative to photodiode detectors. The response time is dependent on the size of the sensor-element. By way of example radial thermopiles with apertures of 19 millimeters (mm) and 200 mm have response times of approximately 1 second and 30 seconds respectively. Spectral response of the thermopile sensors depends on the absorption spectrum of the sensor. With a suitable choice and configuration of the sensor, the spectral response can extend from ultraviolet (UV) wavelengths to far infrared wavelengths. With a sufficient heat sink, thermopile sensors can measure lasers power up to about 10 kilowatts (kW).
One relatively new detector type which has been proposed to offer a temporal response comparable to a photodiode detector and a spectral response comparable with a thermopile detector is based on using a layer of an anisotropic transverse thermoelectric material as a detector element. Such an anisotropic layer is formed by growing the material in an oriented polycrystalline crystalline form, with crystals inclined non-orthogonally to the plane of the layer.
The anisotropic layer absorbs radiation to be measured thereby heating the layer. This creates a thermal gradient through the anisotropic material in a direction perpendicular to the layer. This thermal gradient, in turn, creates an electric field orthogonal to the thermal gradient. The electric field is proportional to the intensity of incident radiation absorbed. Such a detector may be referred to as a transverse thermoelectric effect detector. If the anisotropic layer is made sufficiently thin, for example only a few micrometers thick, the response time of the detector will be comparable with that of a photodiode detector. Spectral response is limited only by the absorbance of the anisotropic material. A disadvantage is that the transverse thermoelectric effect is relatively weak compared to the response of a photodiode.
One transverse-thermoelectric-effect detector is described in U.S. Pat. No. 8,129,689, granted to Takahashi et al. (hereinafter Takahashi). Takahashi attempts to offset the weakness of the transverse thermoelectric effect by providing first and second anisotropic material layers which are grown on opposite sides of a transparent crystalline substrate. In the Takahashi detector, radiation not absorbed by the first layer of anisotropic material is potentially absorbed by the second layer. It is proposed that a reflective coating can be added to the second layer to reflect any radiation not absorbed by the second layer to make a second pass through both layers.
Oriented polycrystalline layers can be deposited by a well-known inclined substrate deposition (ISD) process. This process is described in detail in U.S. Pat. No. 6,265,353 and in U.S. Pat. No. 6,638,598. Oriented polycrystalline layers have also been grown by a (somewhat less versatile) ion-beam assisted deposition (IBAD) process. One description of this process is provided in a paper “Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam assisted deposited yttria-stabilized-zirconia” by C. P. Wang et.al, Applied Physics Letters, Vol 71, 20, pp 2955, 1997.
The above described Takahashi detector allows the anisotropic material layers to remain thin, while increasing the amount of light absorbed, but requires a transparent crystalline substrate polished on both sides, at costs potentially prohibitive for most commercial applications. Further, the Takashi detector arrangement isolates the crystalline substrate limiting the ability to heat-sink the substrate. This limits the power-handling capability of the detector to a maximum power of less than about 10 Watts (W), and may lead to a non-linear response.